Tap changers are used for controlling the output voltage of a transformer by providing the possibility of switching in or switching out additional turns in a transformer winding. In a diverter-type tap changer, the electrical connection between fixed contact and external contact is typically formed by a diverter switch together with a tap selector.
WO2014/124771 shows examples of diverter switches for an on-load tap changer. The diverter switch comprises a main branch and a transition branch arranged alternating between first and second connection points, and an external connection point. The main branch comprises a moving contact adapted to be moved between being connected to the first connection point and connected to the second connection point. The transition branch comprises a moving contact adapted to be switched between being connected to the first connection point and to the second connection point. A control unit is configured to move, upon receipt of a signal indicative of a desire to perform a tap changing, the main branch from the first connection point to the second connection point by performing a switching sequence.
WO94/01878 discloses a tap selector for an on-load tap changer. The tap selector comprises a current connector including two tap selector shafts, a plurality of contact rings electrically connected to the current connector, a plurality of moving contact slidably connected to one of the contact rings. The tap selector further comprises a circular hollow cylinder with a closed circumference surrounding the current connector and the moving contacts, and a plurality of fixed contacts. The contacts being fixed to a wall of cylinder. Each moving contact is adapted to connect one at a time with the fixed contacts, which are placed in the same circular orbit as the contact ring. The moving contacts comprises at least two elongated contact elements in the form of contact fingers extending between the fixed contact and the contact ring and arranged in parallel. The moving contact is slidably connected to the contact ring and is rotatable about a rotational axis coinciding with a central axis of the contact ring. The contact fingers of the moving contact has a first portion adapted to electrically connect to the fixed contacts and a second portion connected to the contact ring. The contact finger has at least one contact area in the first portion for providing electrical contact with the fixed contacts and at least one contact area in the second portion for providing electrical contact with the contact ring. The contact area of the second portion is located where the contact ring is closest to the fixed contact. i.e, where the distance between the contact ring and the fixed contact is shortest.
The currents through the parallel contact fingers of the moving contact causes attraction forces between the contact fingers. Further, opposing currents in the contact areas generate separating forces. In the state of the art moving contacts, the attraction forces and separating forces are in balance. However, for some applications there is a desire to have more compact moving contacts to reduce the size of the tap changer and more particular to reduce the size of the tap selector. To achieve a more compact moving contact, the distance between the fixed contacts and the contact ring has to be reduced. Accordingly, the length of the contact fingers has to be reduced. When the length of the contact fingers is reduced, the attraction force between the contact fingers is reduced and by that the balance between the attraction forces and the separating forces is destroyed. Another problem with reducing the length of the contact fingers is that the cooling area of the moving contact becomes too small to handle high currents. One solution to this problem is to increase the number of parallel contact fingers. However, this is costly and increases the space demand rather than lower it.